According to CR & NL, Canon recently has filed several new telephoto prime lens patents at the USPTO. The range of patents are from 200mm to 800mm.
According to CR, along with EF 600mm f/4 DO IS lens, there will be some new big white telephoto prime lenses coming in 2018. And Canon may also bring EF 200mm f/2L lens to EF 200mm f/1.8L lens.